Plastic molded parts have been produced from thermoplastic melt by the interior gas pressure method. This is described, for example, in DE 37 34 164 A1 or the related U.S. Pat. No. 5,047,183. With this method, melt is first injected into the cavity of the injection molding tool, and then nitrogen is injected. The gas forms a cavity in the interior of the melt; while the melt is cooling, the gas pressure presses it against the cavity walls, so that volume contractions due to cooling can be compensated and the surface of the molded part is free of sink marks.
Another previously known method is based on the use of expanding agents to produce so-called structural foams (TSG). The density of resulting plastic structures decreases from outside to inside. With this method, an expanding agent is admixed with the thermoplastic melt in the region of the plasticizing unit. The injection temperature is determined by the so-called kick-off temperature of the particular expanding agent, since it is desired that the expanding agent should foam only in the mold cavity.
Another injection molding method using a gas is known from DE 30 20 122 C2, or the related U.S. Pat. No. 4,381,272. In the disclosed system, the gas is injected directly into the preplasticizing cylinder, and is mixed with the plastic melt. The mixture of plastic melt and gas is then injected into the mold cavity. A plasticizing screw is used for this process, with a special melt storage feature. The method makes it necessary to meter the gas quantity exactly and to control the instant of gas injection exactly, so as to catch the point of lowest material pressure in the screw. The referenced document discloses a circuit arrangement that has two interconnected timing relays.
Problems exist with each of these techniques in various applications. For example, a disadvantage of the solution proposed by DE 30 20 122 C2 is that axially fixed screws are used, which generally must be specially produced, i.e., being nonstandard components. Another disadvantage is that the method described in DE 30 20 122 C2 is not sufficiently reliable, especially for the precise production of large, expanded molded parts such as plates. In contrast, special expanding agents are required for the TSG technique.
The present invention is directed to a refinement of these previously known injection molding methods. It enhances the reliability of production so that it is possible to produce even large molded parts exactly in a desired mixture of plastic and gas. Furthermore, it should be possible to use the method and its associated apparatus in conventional injection molding machines without significant conversion work.
In general, according to one aspect, the invention features injecting the mixture of thermoplastic melt and fluid into the cavity of an injection molding tool through the translational motion of the plasticizing and injection screw. The fluid is introduced into the preplasticizing cylinder at an axial position where the screw channels of the plasticizing and injection screw are situated at least some of the time. The pressure difference (delta p) between the pressure (PF) of the fluid and the pressure (ps) in the thermoplastic melt is maintained essentially constant while the fluid is being introduced into the preplasticizing cylinder or the volume of the introduced fluid is metered. The of the introduced fluid is metered and/or controlled, so as to maintain the desired pressure difference (delta p).
This procedure achieves the result that the admixture of fluid, especially gas, to the plastic melt is influenced in such a way that the melt-gas mixture can be maintained exactly at a desired consistency. This results from the surprising fact that maintaining a precise pressure difference, e.g. 20 bar, between the higher fluid pressure and the lower melt pressure has a stabilizing effect on the formation of the desired melt-gas mixture.
A first development is characterized in that the pressure (ps) in the thermoplastic melt (1) is maintained at a low level at least while the fluid (4) is being introduced. This tends to assure that the melt-gas mixture will be formed with the desired consistency. The reference quantity for the pressure reduction is the usual pressure prevailing in the screw region during injection molding. According to the invention, this is preferably reduced about 50%. For this purpose, the usual injection molding parameters familiar to a person skilled in the art (e.g. rotational speed of the screw while plasticizing) are appropriately adjusted.
Another development is characterized in that more fluid is added even while the mixture of thermoplastic melt and fluid is being injected into the cavity of the injection molding tool.
The inventive method can operate especially well if a multi-zone screw is used as the plasticizing and injection screw, the fluid being introduced in the region of its screw channels. It is particularly appropriate that the multi-zone screw is a six-zone screw, comprising an input zone, a compression zone, a metering zone, a decompression zone, another compression zone, and an ejection zone.
The above pressure reduction in the region of the screw channels preferably can also be achieved by using a multi-zone screw, such that the decompression zone of the multi-zone screw is situated in the region of the gas input point.
The method advantageously can also be used in conjunction with an injection molding tool with a movable element for changing the volume of the cavity of the mold, that is with a breathing mold. Another possible development is for the injection molding tool to be capable of a stamping process, i.e. a stamping tool is used. A breathing core and a stamping tool also can be combined advantageously. The injection process advantageously is begun with an initial cavity volume of the injection molding toolxe2x80x94the gap between the cavities preferably is 0 mm. The cavity volume is increased during the injection process, and toward the end of the injection process it is reduced again by the execution of a stamping process.
In order to influence the foaming process of the melt-gas mixture in the mold, the invention further provides that this counterpressure can be controlled or regulated in accordance with a given time pattern.
The apparatus for injection molding plastic molded parts from thermoplastic that implements the method comprises the following elements: a preplasticizing cylinder with a plasticizing and injection screw situated therein so as to be movable both rotationally and translationally (axially), to produce a thermoplastic melt. A system for injecting a fluid at an axial injection position of the preplasticizing cylinder is also provided.
According to the invention, the injection fluid system is disposed at an axial position of the preplasticizing cylinder, at which position the screw channels of the plasticizing and injection molding screw are situated at least some of the time, and a melt pressure measurement system provides for measuring the pressure (ps) in the thermoplastic melt in the preplasticizing cylinder. A fluid pressure measurement system measures the pressure (PF) of the fluid. A pressure difference determining system assesses the pressure difference (delta p=PFxe2x88x92Ps) between the pressure (pF) of the fluid and the pressure (ps) in the thermoplastic melt.
A system is also provided to measure and/or meter the volume of the fluid that is introduced into the melt.
Here, too, the injection molding parameters for controlling or regulating the pressure, such as are familiar to a person skilled in the art, are relevant, namely the speed (rpm) of the screw and the axial injection force and injection speed of the screw, and on the other hand, of course, the fluid pressure (gas pressure) itself.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.